The present invention relates to an electromagnetically actuated fuel atomising and metering valve for a heat engine fuel supply device.
A valve of this type substantially comprises a metal body in which is housed an annular electromagnet and a core disposed within the electromagnet, as well as an injection nozzle which is provided with at least one fuel injection orifice and which is fixed to the said body. A valve of this type further includes a shutter member movable between a first position in which it closes the fuel passage through the injection opening and a second position in which it leaves this passage open; the shutter member is normally fixed to a tubular armature which can be attracted by the core and which is provided with a cylindrical outer surface arranged to slide on a corresponding surface of the opening in the valve body.
Valves of the type briefly described have several disadvantages.
Above all, these can have an insufficient speed of response, especially when they have to operate at a particularly high frequency. This disadvantage is due to the structure of the assembly constituted by the shutter member itself, and by the armature which is fixed to it; in fact, because of the shape of these two parts and the members which are necessary to connect them together the mass of the assembly thus formed can be excessively high; moreover, the magnetic flux linkage between the armature and the valve body can be poor because of the high value of the radial clearance (air gap) which must be left between the outer surface of the armature and the inner surface of the opening in the body which constitutes the guide for the movement of the armature itself; in fact, the value of this clearance depends on the working tolerances of the surfaces, which tolerances cannot be made stricter.
Moreover, the valve seal may also be unsatisfactory: this disadvantage is due to small errors in mounting the shutter member proper onto the armature and which are due to the manner in which mounting is performed (normally by plastic deformation of parts of the armature) to achieve this connection; moreover an unsatisfactory seal is also sometimes consequent on an incorrect surface roughness of the active surface of the shutter member and that of the seat with which it comes into contact when the valve is in its closed position.
Finally, small drops of fuel can form on such valves downstream of the injection opening which, by mixing with the flow of fuel and air mixture, which is supplied to the engine can vary the operating conditions thereof, in particular in slow running conditions; this disadvantage is due to the fact that the jet of fuel leaving the injection opening strikes a surface which is formed within the valve nozzle; during the impact between the jet and this surface the fuel is atomised, but part of it can run off tangentially on the surface itself collecting into droplets which can then drop off it and be conveyed into the fuel and air mixture.
Finally, the structure of valves of the type indicated is normally rather complex because of the shape of some of the parts of the valve and the type of connections which are provided for mounting them.